The present invention relates to the preparation of getter materials, and more particularly to methods for cleaning and/or activating getter materials.
Getter materials have been used to selectively sorb gasses. For example, getter material includes zirconium, titanium, tantalum, niobium, hafnium and yttrium and alloys of at least one of these metals with one or more elements selected among the transition metals, Rare Earths and aluminum. Getter materials also include metal alloys, metal compounds (such as oxides and chlorides), and non-metallic materials.
Some getter materials are capable of reversibly sorbing hydrogen and substantially irreversibly sorbing gasses as oxygen, water, carbon oxides and, in some cases, nitrogen. Non evaporable getter materials, also known as “NEG,” are often used for the maintenance of vacuum. The maintenance of vacuum is required, for example, in particle accelerators, X-ray tubes, field-emission flat panel displays, evacuated jackets for thermal insulation (such as in thermal vessels, e.g., Thermos® and Dewar bottles), and the pipes for oil extraction and transportation.
NEG materials can be also employed to remove reactive gasses when traces thereof are present in other gasses that are not reactive to the NEG materials, e.g. inert or noble gasses. An example, NEG materials may be placed within fluorescent lamps, which are filled with noble gasses at a pressure of some tens of mbar, so that the NEG material can remove traces of oxygen, water, hydrogen and other contaminants from the noble gasses. Another example is the purification of inert or noble gasses, particularly for applications in the microelectronics industry.
Miniaturized mechanical or electromechanical devices are generally known in the field as “Microelectromechanical Systems”, or “MEMS”, while the miniaturized devices of optical type are known as “Microoptoelectromechanical Systems” or “MOEMS”. For the sake of simplicity, however, the definition MEMS will be used in the present text also with reference to MOEMS. MEMS generally comprise an active device (the micromechanical, microelectromechanical or optical part) and supplementary parts, enclosed in a sealed cavity, with electrical feed-throughs for power and the transmission of electrical signals.
The latest generation of MEMS is manufactured with technologies derived from the semiconductor manufacturing industry. These technologies include processes such as deposition, masking and etching. The processes can be used to manufacture miniaturized structures and geometries that are not obtainable or practical through traditional manufacturing techniques.
Among the main types of MEMS that are already in commercial use or that are in close-commercial, are: microaccelerometers such as those disclosed in U.S. Pat. No. 5,952,572 to trigger the deployment of automobile air bags; miniaturized mirrors such as those disclosed in U.S. Pat. No. 5,155,778 for use in optical fiber telecommunication systems; arrays of miniaturized mirrors, such as those disclosed in U.S. Pat. No. 6,469,821 and used in the formation of images; and microbolometers, e.g., miniaturized sensors of infrared radiation, such as those disclosed in U.S. Pat. No. 6,252,229.
Several ways for manufacturing MEMS have been proposed, but the most common ones use at least two planar supports made of glass, quartz, a ceramic material (e.g. silicon carbide) or a semiconductor material (silicon is preferred). The active and passive components of the MEMS are then constructed on the planar supports. Most commonly, the active parts are constructed on a first silicon support (e.g. the movable parts of a micromechanical device) while the second support (which may be made up of glass, quartz, a ceramic material or a semiconductor material) is used to enclose the finished device. The two supports are configured to create a cavity, often held at a low pressure or “vacuum,” which houses the MEMS mechanism. That is, the MEMS mechanism is typically enclosed (or “housed”) within a cavity or chamber bounded, at least in part, by the supports. The electrical feed-throughs for transferring the signals between the inside and the outside of the MEMS and for providing power can be provided with respect to either or both supports.
Once all the components necessary for the operation of the MEMS have been manufactured, the supports are bonded together. The microdevice is thus sealed in a closed space and is mechanically and chemically protected from the outside.
Numerous techniques for attaching the two supports together will be collectively referred to herein as “bonding.” A first possibility is welding, either by the application of heat (“simple welding”), pressure (“pressure bonding”), or both. Typically, with welding, a malleable metal such as indium, lead or gold is interposed between the two supports, and is then melted and allowed to re-solidify in the case of simple welding, or pressed between the substrates, in the case of pressure bonding. Pressure bonding, in particular, has not been found to be completely reliable.
Another type of bonding is anodic bonding, used especially in the case in which one of the two supports is made up of glass or quartz and the other of silicon. With anodic bonding, the two parts are heated to a temperature of between about 300-500° C., and a potential of about 1000V is applied between the supports. Under these conditions, there is a migration of positive ions from the support kept at the more positive potential (for example, sodium ions from the glass) toward the support kept at the more negative potential, and a migration of negative ions (for example, oxygen from silicon) in the opposite direction which results in mutual welding at the interface between the two supports.
Another bonding technique is eutectic bonding, wherein a layer of a metal or alloy capable of forming a eutectic composition with the material of at least one of the two supports in interposed between the two supports. A eutectic (exothermic) reaction then causes a localized melting and welding of the two materials.
Finally, another bonding technique includes direct bonding, which comprises the localized high-temperature melting of the material of the supports. Unfortunately, this process generally requires very high temperatures, for example about 1000° C. in the case of the silicon, which may damage the component parts of the microdevice.
Generally, all the types of bonding described above require a previous treatment of the support surfaces to be fixed to each other, because unprepared surfaces may reduce the effectiveness of the welding. One class of treatment is mechanical in nature (e.g., gas-blowing or scrubbing with solid CO2). This treatment can, among other things, remove particles present in the welding area. Another class of treatment is of a chemical type that alters the composition of the surface. This treatment can, among other things, eliminate contaminant species (e.g., oxides). Chemical treatments generally involve applying acidic or basic solutions, or combinations thereof, in sequence to the supports.
MEMS devices often require a particular atmosphere for their operation. For example, microbolometers must be maintained under very low pressure or vacuum conditions, because traces of gasses, if any, could give a convective contribution to the heat transportation in the system which could distort the measurement. MEMS with moving parts are often in vacuums, or low pressure or inert atmospheres. The humidity of the atmosphere must often be controlled because the molecules of water present on the surface of the different parts that comprise the microdevice may give rise to adhesion phenomena or modify the friction between the stationary parts and the moving parts, thus modifying the mechanical characteristics of the system. The control of the inner atmosphere, regardless of pressure (which may vary from vacuum to pressurized), of a MEMS is, consequently, generally extremely important for proper operation.
There are multiple phenomena that tend to deteriorate the quality of the inner atmosphere of a MEMS. Welding is one such phenomenon. Even if properly done, welding between two supports inevitably results in microscopic cracks through which gasses may pass into a cavity housing a device or active element of a MEMS.
A second phenomenon that can contribute to the deterioration of the atmosphere of a MEMS is outgassing. Gasses sorbed from the device and/or the supports tend to be released into the cavity housing the device or active element of the MEMS. MEMS have a very high ratio between inner surface area and cavity volume such that a relatively large concentration of outgassing occurs in a very small volume. Accordingly, though this problem is common to practically all devices under vacuum or in a controlled atmosphere, outgassing is particularly felt in the case of MEMS. Even though the inflow of gas into a MEMS cavity due to these two phenomena is small, it tends to be cumulative over the life of the device, so that over long periods of time, the aggregate effect is substantial.
A third phenomenon that can contribute to the deterioration of the atmosphere of a MEMS finds its origin during the manufacturing process of the MEMS. More particularly, the welding of the two supports frequently requires high temperatures, from some hundreds of degrees up to about 1000° C. or more, resulting in the release of relatively large amounts of gasses which can become entrapped within the cavity of the MEMS devices.
To obviate these problems, the MEMS manufacturers use NEG materials to absorb gasses. However, in contrast to traditional manufacturing techniques, wherein the different parts of a device are separately manufactured and finally assembled, with MEMS, generally all the components of a device are manufactured in series on one or two supports. This raises the general problem of compatibility between each material deposited on a support with all the subsequent manufacturing steps. When there is an incompatibility between a material and a process step, the material already deposited must be temporarily protected, for example with polymeric materials that are removed after the critical operation has been made. However, this process increases the complexity and time of manufacture and, as such, consequently increases cost.
The existence of at least some of the problems related to MEMS manufacture is acknowledged, for example, in U.S. Pat. No. 6,252,229 of Hayes et al. In particular, it has been acknowledged in the prior art that NEG, once deposited on a support, needs to be protected until the end of the manufacturing process due to its high chemical reactivity. In particular, in the prior art, the treatment of a support with caustic materials (e.g. acidic or basic baths) after NEG material is deposited thereon was considered to be destructive to the NEG.
This problem is the subject of the published patent application US-A1-2003/0138656 of Sparks, which discloses a method for manufacturing a support with a getter deposit, particularly for use in MEMS. Sparks expressly teaches that the getter is a fragile component of the system, and must be protected from the moment it is manufactured until the end of the manufacturing cycle of the MEMS. For this purpose, Sparks suggests that the getter material be covered with a layer of a few nanometers of a noble metal, such as gold, which is inert and resistant to gasses and to chemical reagents used in the different manufacturing steps of the MEMS which typically follow the getter deposition step. Sparks teaches, as a final or “bonding” step in the manufacturing process, the high temperatures of the bonding process cause the protective layer of noble metal to diffuse in the underlying material, thereby exposing the getter surface to the atmosphere within the cavity of the MEMS.
The above mentioned U.S. Pat. No. 6,252,229 of Hayes et al. proposes a manufacturing process that includes a double-step bonding. The double-step bonding includes a first “pressure bonding” step along a continuous closed line around the cavity to obtain a gas first bond, and a second bonding step, for example of anodic bonding, creating a second bond outside of the first bond, to create a mechanically resistant welding of the two supports. This process is said to reduce leakage into the cavity of the MEMS and, as such, is said to eliminate the need for a getter in the cavity. However, Hayes et al. do not address the problems of outgassing or gasses trapped during the manufacturing process. Furthermore, the two step process is much more complicated and expensive than a single step bonding process.
U.S. Pat. Nos. 6,621,134 and 6,635,509 disclose processes wherein the MEMS are manufactured starting from a single support, thus avoiding the problem of the bonding with a second support. These patents indicate the presence of a getter material. However, since the step of formation of the NEG deposit takes place almost at the end of the process, the getter is deposited externally to the cavity. Small openings are provided in the walls of the support to allow gettering through the wall openings. The amount of getter available through the holes is extremely limited, and the getter life is reduced by its placement outside of the cavity.
U.S. Pat. No. 5,701,008 discloses a microbolometer manufactured starting from two supports and containing a getter material. As far as the description of the manufacturing process is concerned, however, this document refers to a previous U.S. Pat. No. 5,433,639, which relates to the manufacturing process of a sensor of infrared radiation of traditional type (not a MEMS) wherein the different components are manufactured in parallel and assembled at the end. As such, the process of U.S. Pat. No. 5,433,639 is not directly applicable to U.S. Pat. No. 5,701,008, at least with respect to the integration of the getter within a cavity, and therefore this last document is of limited value in addressing the above-identified problems.
U.S. Pat. No. 6,590,850 mentions the general use of a getter in a MEMS and discloses the location thereof, but it does not disclose the manufacturing process of the devices and consequently does not mention how to introduce the getter therein. U.S. Pat. No. 5,952,572 is even more indefinite, mentioning only the use of a NEG, a combination between titanium and an alloy Zr—V—Fe, without disclosing either the location of the getter in the cavity or, even less, the step of introducing the NEG in the cavity.
The efficient integration of getter material into a chamber of a MEMS during the manufacturing process is still an open problem, and that the solutions proposed up to now are complicated, ineffective and expensive.